This invention relates in general to a machine for handling sheet metal and more particularly to a machine for feeding or advancing a strip of sheet metal in measured increments.
Sheet metal finds widespread use in manufactured products, particularly in housings for appliances and in cabinets of one sort or another. Most of this sheet metal comes from large coils produced at rolling mills. But usually the width in which the coiled metal sheet is furnished does not correspond to a dimension required for a manufactured product, and never does the length. Typically fabricators slit the sheet longitudinally and shear it transversely to provide panels of a size suitable for further fabrication into manufactured products.
Slitting, when it is performed, presents little difficulty. The knives of the slitter are set in the proper locations, and the metal strip is simply passed through the knives as it is withdrawn from the coil.
On the other hand, shearing presents a greater challenge in that each transverse cut or shear must occur while the sheet metal strip is at rest. This requires advancing the strip a prescribed distance, stopping it, then shearing it, and then repeating the foregoing. Two types of feeding machines have evolved for effecting incremental advances. One relies on pinch rollers through which the sheet metal strip passes. The rollers start and stop, rotating precisely the same amount each time. The rollers, however, grip the metal strip only in very limited areas, and are susceptible to slipping, particularly in the presence of oils which one invariably finds on coiled steel strip. Of course, any slippage detracts from the precision which is so necessary in producing panels of equal size.
The other type of feeding machine has gripping units which grip the strip and move with the strip as the strip advances, and indeed the strip remains gripped at the same locations during the entire incremental advance. The areas along which the strip as gripped are quite large, so slippage is less likely to occur. Thus, this gripper-type of machine advances the strip with considerable precision. However, after each advance the gripping units must return to their original positions to again grip the strip for the next advance. The shear cuts the strip transversely while the strip is at rest, but even so the shearing takes less time than the return of the gripping units, so that machines with movable gripping units do not operate as rapidly as feeding machines with pinch rollers. U.S. Pat. No. 3,753,522 entitled Sheet Transferring Device and Method discloses a gripper-type feeding machine, that is one with movable gripping units.
A gripper-type feeding machine actually grips the sheet metal strip along pads having their surfaces ground smooth to avoid marring the metal sheet where they clamp down on the strip. From time to time these pads must be removed to refinish their ground surfaces. Typically, machine screws secure the pads and of course must be removed to release the pads. This is a time consuming procedure.
In a gripper-type feeding machine, the gripping units are typically advanced and retracted with an endless chain that passes over sprockets at each end of the paths taken by the gripping units. The chains transmit substantial forces during the advance of the strip and in time stretch and become loose.